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International Journal of Control

ISSN: 0020-7179 (Print) 1366-5820 (Online) Journal homepage: http://www.tandfonline.com/loi/tcon20

Interactive smart battery storage for a PV andwind hybrid energy management control based onconservative power theory

Marcelo Godoy Simões, Tiago Davi Curi Busarello, Abdullah Saad Bubshait,Farnaz Harirchi, José Antenor Pomilio & Frede Blaabjerg

To cite this article: Marcelo Godoy Simões, Tiago Davi Curi Busarello, Abdullah Saad Bubshait,Farnaz Harirchi, José Antenor Pomilio & Frede Blaabjerg (2016) Interactive smart batterystorage for a PV and wind hybrid energy management control based on conservative powertheory, International Journal of Control, 89:4, 850-870, DOI: 10.1080/00207179.2015.1102971

To link to this article: http://dx.doi.org/10.1080/00207179.2015.1102971

Accepted author version posted online: 01Oct 2015.Published online: 04 Nov 2015.

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INTERNATIONAL JOURNAL OF CONTROL, VOL. , NO. , –http://dx.doi.org/./..

Interactive smart battery storage for a PV and wind hybrid energy managementcontrol based on conservative power theory

Marcelo Godoy Simões a, Tiago Davi Curi Busarellob, Abdullah Saad Bubshaita, Farnaz Harirchia, JoséAntenor Pomiliob and Frede Blaabjerg c

aDepartment of Electrical Engineering and Computer Science, Colorado School of Mines, Golden, CO, USA; bSchool of Electrical and ComputerEngineering, University of Campinas, Campinas, Brazil; cInstitute of Energy Technology, Aalborg University, Aalborg, Denmark

ARTICLE HISTORYReceived December Accepted September

KEYWORDSStorage; wind energy; solarenergy; asymmetricmultilevel inverter

ABSTRACTThis paper presents interactive smart battery-based storage (BBS) for wind generator (WG) and pho-tovoltaic (PV) systems. The BBS is composed of an asymmetric cascaded H-bridgemultilevel inverter(ACMI) with staircase modulation. The structure is parallel to the WG and PV systems, allowing theACMI to have a reduction in power losses compared to the usual solution for storage connected atthe DC-link of the converter for WG or PV systems. Moreover, the BBS is embedded with a decisionalgorithm running real-time energy costs, plus a battery state-of-chargemanager and power qualitycapabilities, making the described system in this paper very interactive, smart and multifunctional.The paper describes how BBS interacts with the WG and PV and how its performance is improved.Experimental results are presented showing the efficacy of this BBS for renewable energy applica-tions.

1. Introduction

The power delivery industry faces the continuous growthof electrical energy consumption. As society needs moreimprovements in technology and in social advancements,more electrical energy is needed. Therefore, new electri-cal power resources and new ways of integration to thetransmission or distribution grid are required. Recently,renewable energy sources have been applied as a long-term and promising solution to such energy challenges.Currently, distributed generation (DG) systems, whichare composed of both renewable and non-renewableenergy micro-sources, have been integrated into thepower system’s distribution level (Bhende, Mishra, &Malla, 2011; Kroposki et al., 2006). DG has a smallersize compared to traditional power plants. Such powergeneration units associated with their energy manage-ment and demand side control techniques are normallycalledmicrogrids, and when such microgrids have a highdegree of control, integration with the utility and users,and several functionalities, they are called smartgrids(Chakraborty, Weiss, & Simoes, 2007).

It is very important to design appropriate power elec-tronics for integration of renewable energy sources to thegrid. (Kroposki et al., 2006). DG systems such as wind,micro-turbine, IC engine or flywheel storage, generateACoutput voltage usually with variable frequency. Therefore,

CONTACT Marcelo Godoy Simões mgs@mines.edu

theseDGs need anAC–DC converter to surpass the inputdistortion and frequency variation effects before mergingto an AC-compatible grid voltage. Likewise, for DG sys-tems with DC output voltage (such as photovoltaic (PV),fuel cells or batteries), a DC–DC converter is typicallyneeded to boost the DC voltage level to the appropriateDC level (Carnieletto, Brandao, Suryanarayanan, Farret,& Simoes, 2010; Lute, Simoes, Brandao, Durra, & Muy-een, 2014). In both cases, once an appropriate DC voltageis achieved from converters, a grid-connected inverter(GCI) module is used in order to convert the primed DCvoltage to grid-compatible AC power (Harirchi, Simões,Al-Durra, & Muyeen, 2015; Harirchi, Simões, Babak-mehr, Al-Durra, & Muyeen, 2015). Finally, an outputfiltering module filters the AC output of the inverter(Reznik, Simões, Al-Durra, & Muyeen, 2014).

The most promising renewable sources are wind andsolar energy. Both of them may operate in grid-tied orstand-alone conditions. Wind stand-alone systems areused to supply isolated loads requiring energy storageto manage the variable load demand. For such systems,the control scheme is designed to perform the man-agement of power during fluctuation of wind and loaddemand (Barote, Marinescu, & Cirstea, 2013; Bhende,Mishra, & Malla, 2011; Lagorse, Simoes, & Miraoui,2009). Power electronic inverters are used to controlactive/reactive power, frequency, and support grid voltage

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INTERNATIONAL JOURNAL OF CONTROL 851

during faults (Angela, Liserre, Mastromauro, & Aquila,2013; Li, Haskew, Swatloski, & Gathings, 2012).

Storage systems are very important in order to supportPV andWG integration, since wind and solar energymaynot fully satisfy the instantaneous power balance. Thereare several configurations to store energy, such as batter-ies, compressed air, flywheel, supercapacitor and pumpedhydro. They have different power density, volume andtime response. So, theway the energy is stored depends onthe application. There are also differences between powerand energy storage concepts. Storage for power applica-tions is designed to supply power for a long timescale(hours). Pumped hydro and batteries are examples ofpower storage suited for bulky systems (Chakraborty,Simões, & Kramer, 2013). On the other hand, storagefor energy applications is demanded when there is aneed for a high time response. Supercapacitors may sup-ply a great amount of energy in a short time comple-menting power quality (PQ) requirements, and slowerstorage systems are not capable of having a comparableperformance.

Battery-based storage (BBS) combined to wind WGand PV is commonly found in real-world applications.The literature demonstrates the efficacy of using BBSto support the wind generator system (WGS) randomnature (Barote, Marinescu, & Cirstea, 2013; Bhuiyan &Yazdani, 2009; Li, Joos, & Belanger, 2010). A BBS withWGS for a single-phase stand-alone structure is describedby Li et al. (2010) and Bhuiyan and Yazdani (2009) andthey present another stand-alone system containing aWG, a hydro generator and a BBS. The goals are mainlyto achieve the maximum power-tracking and to controlthe magnitude and frequency of the output voltage. Amultimode control for WG aggregated to a BBS unit forremote applications is presented by Barote, Marinescu,and Cirstea (2013).

This paper presents an alternative way of connectingthe BBS with the WG and PV. Most of the past workdescribed in the literature has a BBS connected parallel tothe DC-link which interfaces the generator to the grid orloads. The proposed structure of this paper is to connectthe BBS in parallel to the whole system by using a DC–AC converter. In this paper the authors propose the useof asymmetric cascade multilevel inverter topology withstaircase modulation. Three battery banks are used withdifferent capacities for those batteries.

The use of the multilevel inverter guarantees a reduc-tion in power losses compared to the conventional struc-ture. In storage systems, any reduction in power lossessupports an increase in the time between charging cycles,with an overall improvement of storage response time.Moreover, the point of connection allows the BBS to per-form ancillary services independent of the PV and WGsystem.

The asymmetric cascadedH-bridgemultilevel inverter(ACMI) embedded with the staircase modulation allowsthe operation of the upper cell, which transfers the majorpart of the battery power, to be switched to 60 Hz. Thislow switching frequency contributes to the reduction ofthe power losses.Moreover, theACMI allows aminimisa-tion in the output filter volume due to the reduced voltagetotal harmonic distortion (THD) in the output voltage.The control strategy is based on the conservative powertheory (CPT) (Busarello & Pomilio, 2015).

2. The battery storage system

Figure 1 presents a simplified unifilar (single-line) dia-gram of the proposed three-phase structure, where a gridwith 127 Vrms and frequency of 60 Hz has been used.The BBS is connected in parallel to the WG and PV sys-tem. The ACMI is composed of three series-connectedH-bridge modules with a voltage scaled 1:2:6, presenting19 levels at its terminal voltage. The storage is divided inthree banks, one for each module. The upper module hastwelve 12 V lead acid batteries connected in a series. Sim-ilarly, the middle and lower modules have four and two12 V lead acid batteries, respectively. The banks are con-nected at each DC side of the H-bridge module. TheWGsystem is composed of a permanent magnet synchronousgenerator (PMSG), and it is connected to the grid througha back-to-back system. The PV system is composed of asolar array, an interleaved boost converter, plus one H-bridge converter. A nonlinear load, in this analysis madeby a diode rectifier with an LC filter at the DC side, is con-nected to the PCC (point of common coupling).

2.1 The control strategy

Figure 2 shows the control strategy applied to the BBS. Atthe PCC voltage, the WG system output, the PV systemoutput and the load current are individually measuredand sent to the CPT real-time calculation module. TheCPT calculation gives the current reference to be usedin the BBS. A phase-locked loop (PLL) (Chung, 2000)supplies information about the PCC voltage phase angle.Once the current reference is defined, it is compared tothe BBS output measured current, and the control signalis sent to the current controller. The current controlleris a proportional-integrator (PI) with PCC voltage feed-forward. The AMCI switch commands are modulated bystaircase modulation. A decision algorithm based on theprice of energy, on the state-of-charge (SOC) curve andon the power requirement decides the operation mode ofthe BBS.

2.2 The staircasemodulation

The possibility of the upper module switching to 60 Hzis due to the staircase modulation, also known as near-

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852 M. G. SIMÕES ET AL.

Figure . Simplified diagram of the proposed three-phase structure.

est level modulation (Perez, Rodriguez, Pontt, & Kouro,2007). Figure 3 presents the principle of it. The ref-erence is sinusoidal, and the pattern for each ACMImodule is different. The reference for the upper mod-ule is obtained by comparing the reference to the DC

value. Its result is the switching pattern for the uppermodule. The reference for the middle module is obtainedby subtracting the sinusoidal reference from the switch-ing pattern for the upper module. The resulting signalis then compared to another DC value. This process

Figure . Control strategy applied to the BBS.

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INTERNATIONAL JOURNAL OF CONTROL 853

Figure . Principle of operation of the staircase modulation.

Figure . Voltage and current across a switch during a turn-ontransition.

continues for the next module. Noticeably, the upper cellswitches to 60 Hz. Moreover, the resulting output signalis close to the reference signal, indicating the possibil-ity of a reduction in the volume of the output filter. Thestaircasemodulation has the disadvantage of presenting aregenerative process in the smaller voltage cells depend-ing on the fundamental component amplitude and itmustbe avoided (Rech & Pinheiro, 2007).

The reduction in power losses proposed in this papermay be verified. The switching losses are directly propor-tional to the voltage and current across a switch and alsoacross the switching frequency. Figure 4 illustrates thevoltage and current across a switch during a turn-on tran-sition. The power losses (Psw) are shown by the dashedarea. The result is the higher the switching frequency,the higher the number of transitions and the higher thepower losses. In a conventional pulse width modulation(PWM) converter with 10 kHz switching frequency, thenumber of commutations in a 60 Hz grid-cycle is around300.

Regarding Figure 3, the voltage across the upper mod-ule has a 120 Hz frequency waveform. Therefore, the

upper module transistors present four voltage and cur-rent transitions in a 60 Hz grid-cycle and once the uppercell process approximates 80%of the power, the reductionin losses is evident.

2.3 Decision algorithm

Figure 5 presents the flow chart of the decision algorithmbased on the following rules about how the lead acid bat-tery SOC is evaluated. If the battery needs to be charged,then the price of the energy is evaluated. If its value isconsidered feasible, the BBS charges its batteries with theunit power factor. The process of charging the batter-ies is called mode 1. The threshold voltage to charge thebanks is 11.4 V for each battery. If the batteries do notneed to be charged, the price of the energy is evaluated. Ifthe price of the energy is considered high, then the BBSsells energy to the grid at nominal power; otherwise, theBBS supplies power to compensate for the intermittentbehaviour of the PV and WG systems together. The pro-cess of selling energy to the grid with the unit power fac-tor is called mode 2, while compensating for renewablebehaviours is called mode 3. When the BBS either injectsenergy into the grid or charges its batteries, a PQ require-ment may be applied. In this work, the PQ requirement issimplified by the fact that the grid current must presentTHDi lower than 5%, but a more complex and evolvingPQ index performance can also be implemented. There-fore, the BBS injects active power and charges its batteriesand also operates as an active filter simultaneously. Theprocess of injecting active power with PQ improvementis called mode 4, and the process of charging the batter-ies with PQ improvement is called mode 5. There is alsoa mode in which the BBS operates exclusively as a shuntactive filter. This mode is performed when the battery isat one of the two threshold points, and the price of energyis high. This mode is called mode 6.

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854 M. G. SIMÕES ET AL.

Figure . The flow chart of the decision algorithm.

2.4 Batterymodel

In order to design integrated battery-based power elec-tronics, a very good battery behaviouralmodel is requiredwhich may include several parameters like SOC, termi-nal voltage, cell temperature, internal pressure and so on.There are many battery models in the literature (Chen

& Rincon-Mora, 2006; Gao, Liu, & Dougal, 2002; Jackey,Plett, & Klein, 2009; Li, Mazzola, Gafford, & Younan,2012; Plett, 2004; Salameh, Casacca, & Lynch, 1992;Wanget al., 2011). So, choosing a more complex or a moresimplified model depends on the application. In a typi-cal battery model, the applied current is the input, andthe terminal voltage is the output (Chen & Rincon-Mora,

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INTERNATIONAL JOURNAL OF CONTROL 855

2006; Gao, Liu, & Dougal, 2002). However, in this workit was necessary to reverse the input–output relationshipof the current and voltage, because the modelling of theBBS requires a voltage source as the input while the sys-tem operates in current-mode control in respect to thepower flow to the utility grid. As described in the pre-vious section, the decision algorithm makes use of theSOC curve in order to prescribe the operation mode.Therefore, a third-order battery model is used because itcan guarantee the correct operation of the BBS. Figure 6presents a typical SOC curve, obtained through a sim-ulation (Sato & Kawamura, 2002), with two thresholdpoints. The threshold points are the ones used in ourdecision algorithm, and their locations on the SOC curvewere chosen using the criterion of the spinning reserve(Chakraborty, Simões, & Kramer, 2013).

Figure 7 presents the third-order model of a lead acidbattery (Wang et al., 2011) in which Em is the open-circuitvoltage, R1 is the overvoltage resistance, C1 is the over-voltage capacitance, R2 is the internal resistance, IP(VPN)

is the parasitic current in function of PN voltage, mainlycaused by self-discharge, andR0 is the terminal resistance.

The model may be divided into three parts: (1) themain branch, composed of Em, R1, R2 and C1; (2) the par-asitic branch, composed of the block called IP(VPN); and(3) the terminal branch, composed of the R0.

The battery extracted charge is given by (1).

Qextr (t ) = Qextr_ini −∫ t

0im (τ ) dτ (1)

where Qextr_ini is the charge computed previously.The battery total capacity is given by (2).

C (i,Temp) = kC0

1 + (k − 1)(

iinom

)δ

(1 − Temp

Tempf

)ε

(2)

where k is themultiplier gain,C0 is the no-load capacity at0 °C, Temp is the electrolyte temperature in °C ( f meansfinal), δ, ε are constants and inom is the battery nominalcurrent.

The SOC is given by (3).

SOC = 1 − Qextr

C(i,Temp

) (3)

The depth of discharge (DOD) is given by (4).

DOD = 1 − Qextr

C(iavg,Temp

) (4)

By using the previous equations, the battery modelparameter can be determined.

The overvoltage resistance is given by (5).

R1 = −kR1ln (DOD) (5)

where kR1 is a constant.The overvoltage capacitance is given by (6).

C1 = τ1

R1(6)

where τ1 is main branch constant time.The internal resistance is given by (7).

R2 = k2e(k21(1−SOC))

1 + e(k22 i

inom

) (7)

The terminal resistance is given by (8).

R0 = R00 [1 + ka (1 − SOC)] (8)

where R00 is the terminal resistance at SOC = 1.The parasitic current is given by (9).

IP (VPN) = VPN kpnexp[VPN/

(τpn + 1

)kp

+ k3(1 − Temp

Tempf

)](9)

where τn is the constant time of the parasitic branch. Thebattery terminal voltage is given by (10).

vt = Em + imZeq + iRo (10)

where Zeq is given by (11).

Zeq = R1 + R2 + sR1R2C1

sR1C1 + 1(11)

And the im and i are related as (12).

IP = im − i (12)

Although a state-space formulation leading to a trans-fer function expansion for the battery system can bedeveloped as indicated in Hafsaoui et al. (2010), theauthors of this paper preferred the equation orientedmodel for a circuit-based simulation in PSIM.

The battery capacity is found by looking into the man-ufacturer’s datasheet, and the constants k presented in theprevious equations are not easy to determine, but they canbe found through specific experimental tests; such proce-dure is beyond the scope of this paper (Mandal & Cox,2010; Plett, 2007).

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856 M. G. SIMÕES ET AL.

Figure . The SOC curve and the threshold points used in the decision algorithm.

The battery model is used to understand how the bat-tery behaves. Based on the behaviour, the design of theBBS takes into account the interested aspect. In this case,the SOC curve is used. During operation, the currentpoint of the SOC curve is estimated in order to be usedas input information for the decision-taker algorithmthrough Equations (1)–(3).

2.1.4 BBS current reference

Themethodology described here is presented for a single-phase grid, but it is extended to a three-phase grid, withor without four wires without any loss of generality. Eachone of the six modes of operation has its current refer-ence. Themode 1 BBS current reference is given by (13).

i∗BBS1 (t ) = PchargeVPCC

sin (ωt + ϕ − π) (13)

where ϕ is the PCC phase angle supplied by the PLL, andPcharge is the power reference to charge the batteries.

Similarly, themode 2 BBS current reference is given by(14):

i∗BBS2 (t ) = PnomVPCC

sin (ωt + ϕ) (14)

wherePnom is the nominal power of the storage. Themode3 operates according to the difference between the nom-inal power of the storage and the WG and PV instanta-neous output power. Therefore, the active power refer-ence for the BBS is given by (15):

P∗BBS (t ) = 1 − PPV (t ) − PWG (t )

(pu

), (15)

where PWG and PPV are the WG and PV system outputpower. The right side of (15) has three parts. The first isa unit because the equation is written per unit (pu). Thesecond and third are PWG and PPV, obtained by measure-ment.

The proposed BBS operates only with the active power.However, the WG and PV system may supply more thanjust active power, it might supply reactive power and alsoharmonic currents. Therefore, for this study the BBSmust

Figure . Third-order model of lead acid batteries.

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INTERNATIONAL JOURNAL OF CONTROL 857

extract only the active power information from the WGand PV output power; that is how the CPT is used. Theauthors will investigate further uses of CPT for improvedextra functionalities in the future.

According to the CPT, for a given voltage v(t ) and acurrenti(t ), the active current for one phase is given by(16):

iactive (t ) =1T

∫ t+Tt v (t ) i (t ) dt

vrmsv (t ) (16)

Therefore, the WG output active current is given by(17):

iWG_active (t ) =1T

∫ t+Tt vPCC (t ) iWG (t ) dt

VPCCvPCC (t ) ,

(17)where VPCC is the root mean square (RMS) value of thePCCvoltage. TheWGsystemoutput active power is givenby (18):

PWG (t ) = 1T

∫ t+T

tvPCC (t ) iWG_active (t ) dt (18)

Similarly, the output active current of the PV system isgiven by (19):

iPV_active (t ) =1T

∫ t+Tt vPCC (t ) iPV (t ) dt

VPCCvPCC (t )

(19)And the output active power of the PV system is given

by (20):

PPV (t ) = 1T

∫ t+T

tvPCC (t ) iPV_active (t ) dt. (20)

The mode 3 BBS current reference is given by (21):

i∗BBS (t ) = P∗BBS

VPCCsin (ωt + ϕ) . (21)

The modes 4–6 deal with active filtering. Therefore, inorder to obtain the current reference for these modes, theCPT is also used. The BBS active filter current reference isgiven by (22), which must be added to mode 4 and mode5 references and used alone in mode 6 as a current refer-ence.

iBBS_AF (t ) = iload (t ) −1T

∫ t+Tt v (t ) iload (t ) dt

vrmsv (t )

(22)

3. System implementation

The three-phase system presented in Figure 1 was sim-ulated in PSIM. In order to make the simulation morerealistic and to evaluate the system’s dynamic behaviour,the BBS, the PV and the WG were simulated by usingthe switching converters. Therefore, a brief descriptionon how the WG and PV systems were designed and sim-ulated results are presented in the following section.

3.1 Wind generator system

This section describes the WG system (Li & Chen, 2008;Simoes, Bose, & Spiegel, 1997). The power transfer andcontrol of a wind turbine system is based on PMSGand connected to the three-phase grid using a back-to-back converter. The machine side converter is controlledto extract the maximum power from wind using vectorcontrol. The grid side controller is designed to controlthe power flow using α–β reference frame. The powertransfer strategy is proposed to control the flow of thepower between the WT and the grid. Figure 8 presentsthe WG system and its control strategy. When the windgoes through the turbine, a mechanical torque (TW ) isapplied to the PMSG. Measured wind speed (for exam-ple with anemometer) is used to get the optimum rotorspeed required to achieve the maximum power from thewind turbine. The speed of the rotor is measured in orderto compensate for the error of the controller and the ref-erence of the direct axis current is zero. The PIs of thed-axis and q-axis can be designed using a small-signal(average)model, and the d–q reference voltages are trans-formed using Clarke and Park transformations to formthree-phase reference voltages to command the switchesof the converter using the sinusoidal pulse widthmodula-tion (SPWM) method. The DC-link voltage is controlledin a constant value by the grid side converter.

.. Wind turbinemodelThe air motion across the wind turbine producesmechanical power expressed as (23).

Pw = 12Cp (λ, β)Awρairv

3W , (23)

where the power coefficientCp is a function of pitch angleand tip speed ratio is expressed as (24) (Lubosny, 2003,pp. 80–81)

Cp (λ, β) = c1(c21γ

− c3β − c4βx − c5)e−

c6γ (24)

The coefficientsγ , x and c1 − c6 in (25) can be definedin different ways based on the type of turbine rotor (Erich

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858 M. G. SIMÕES ET AL.

Figure . Wind turbine model and control scheme.

& Renouard, 2013). The coefficients defined in Lubosny(2003) are used for the wind turbine model where

c1 = 0.5, c2 = 116, c3 = 0.4, c4 = 0, c5 = 5,c6 = 21, x = 0, (25)

1γ

= 1γ + 0.08 ∗β

− 0.0351 + β3 (26)

The tip speed ratio is defined as (27):

λ = RWωr

vW, (27)

where RW is the radius of the turbine’s blades, ωr is theturbine rotor speed and vW is the average wind speed.

.. PSMGmodelThe d–q equivalent circuits of PMSG are described(Krishnan, 2010, pp. 226–260). The core loss is ignoredin the circuits assuming high core resistance. The equa-tions of the d-axis and q-axis in rotating reference frameare given as (28) and (29).

v rds = −Rsid − Ld

ddt

id − ωeLqiq (28)

v rqs = −Rsiq − Lq

ddt

iq + ωeLdid + ωeψm (29)

The instantaneous power is given by (30):

Pi = 23

[v rdsid + v r

qsiq]

(30)

Replacing (28) and (29) in the power equation andfrom the air gap power, the electromagnetic torque is

given by (31):

Te = 34

p[ψm + (

Ld − L − q)id

]iq (31)

Then, the rotational angular speed is expressed as(32):

ddt

ωr = 1J

(TW − Te − Bωr) , (32)

whereTW is the torque produced by thewind turbine, andTe is the turbine equivalentmoment of inertia, andB is thefriction coefficient. The angular electrical speed is relatedto the rotor speed by (33).

ωe = p2

ωr, (33)

where p is the number of poles of the synchronous machine.

.. Machine side controllerThe purpose of the machine side controller is to track theoptimum point of the rotor and extract the maximumpower existing in the turbine. For a given wind turbine,themaximumpower occurs at themaximumpower coef-ficient (Cp,max) of the turbine. For a given wind speed,there is an optimum rotor speed that gives the optimumtip speed ratio (34):

λopt = RWωr,opt

vW(34)

A simple approach to extract themaximumpower is tocalculate the optimum tip speed ratio ahead of time andthen calculate the required rotor speed for any measured

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INTERNATIONAL JOURNAL OF CONTROL 859

Figure . α-axis control loop.

wind speed as (35).

ωr,opt = vWλopt

RW(35)

The maximum power, though, occurs at the opti-mum speed for every wind speed (vW ). As the windspeed changes from one point to another, the optimumpoint changes to a different value. A controller must bedesigned to follow the reference speed, and different tech-niques can be applied using a search technique that doesnot needmeasurement of the wind speed or use the directrelationship as in (35).

.. Grid side controllerThe purpose of the grid side converter is to inject elec-trical power in the grid produced by the generator with agood PQ. Moreover, the DC-link voltage control is alsocommanded by the grid side converter. The converteroutput current flows through the grid. Therefore, it isdesirable that this current has a sinusoidal waveform.Figure 9 presents the α-axis control loop used in the gridside converter, and a similar closed loop is used for theβ-axis. The grid converter output current is controlledby the inner loop and has a faster response than the DC-link loop. Therefore, the inner loop is considered unitarywhen the DC-link voltage control is designed. The powercalculation uses the active and reactive power referencesto produce the current references, which in turn will bemultiplied by the DC-link controller output signal. Thenthe signal is used to command the inverter.

The PWM block could be assumed to have a gain ofone. The power calculation block is given by (36) (Yaz-dani & Iravani, 2010, pp. 163–164)

iα, power (s) = 23

vsα

v2sα + v2

sβPref (s) + 2

3vsβ

v2sα + v2

sβQref (s) .

(36)The inverter plus the filter connecting the inverter to

the grid can be expressed as (37).

IF (s) =VDC

/2

sL + R. (37)

Table . PMSG parameters and wind turbine specifications.

PMSG

Rs (stator resistance) .�

Ld (d-axis leakage inductance) . mHLq (q-axis leakage inductance) . mHψm (leakage inductance) . WbP (number of poles of machines)

Wind turbineNominal output power kWBase wind speed m/sBase rotor speed rpm

The DC-link plant is given by (38):

VDClink (s) = 2sCDCVs,pk

. (38)

The current controller is composed of a phase-leadand phase-lag controller and the voltage control is aPI. Both are designed using classical frequency-responsetechniques. The designed controller transfer functions forthe current and voltage are given by (39) and (40).

Ci (s) = s2.804233 · 10−4 + 1s23.840346 · 10−11 + s1.377492 · 10−6 , (39)

CvDC (s) = 0.232s + 0.03

s(40)

Figures 10 and 11 present the Bode diagrams for thecurrent loop and for the voltage loop, respectively.

The PMSG parameters are similar and presented inTable 1 (Xia, Ahmed, & Williams, 2011). The wind tur-bine has the optimum wind speed of 200 rpm at 10 m/srated wind speed.

The performance of the machine and grid side con-verter is evaluated. Thewind turbine tracks themaximumpower point (MPP) as shown in Figure 12. The DC-linkvoltage is kept constant at 1400 V by the grid side con-verter.

Figure 13 presents the DC-link voltage in steady-statecondition. It is constant and controlled at 1400 V.

Figure 14 presents the three-phase PCC voltage andtheWG three-phase output current in a steady-state con-dition. The current is sinusoidal and in phase with thePCC voltage.

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Figure . Bode diagram for the current loop.

3.2 PV system

This section describes the PV system designed. Theschematic of the proposed power electronic interface fora grid-connected PV-based system and its control strat-egy is shown in Figure 15. A floating four-leg interleavedboost converter (FIBC) and a smart grid-connectedinverter (SGCI) were used. The FIBCs for PV and fuel cellapplications are used because of their high voltage gain,high efficiency, low input current ripple and smaller com-ponents (capacitors and inductors) (Lute et al., 2014). Inthis paper, a type-III compensator is designed in order tocontrol the output voltage of the FIBC and support a highgain and constant DC-link voltage for the SGCI in thefull FIBC–SGCI interface. In order to limit the effect ofthe harmonics, the SGCI is connected to the grid through

an LCL filter module. The proposed control techniquefor the SGCI is based on d − q theory, where a smoothtransition method is applied in order to switch betweenGC and islanding modes.

.. Designing a controller for FIBCA controller for DC–DC FIBC with an input voltage of25 V (which is the output voltage of PV modules) andan output voltage of 380 V is presented in this section.A type-III controller is selected to control this converter,and the controller contains a closed-loop error ampli-fier circuit, a power stage and a PWM unit. The PWMhas been used in order to apply the output control signalfrom the compensator to the FIBC switches. The erroramplifier compares the converter output voltage with a

Figure . Bode diagram for the voltage loop.

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Figure . Measured (red) and reference (blue) rotor speed in rad/s.

reference voltage in order to produce an error signalthat is used to adjust the duty ratio of the switch. Thesmall-signal transfer function of the type-III amplifier isexpressed in terms of its input and feedback impedancesZi and Z f and given by (41):

G (s) = Vc (s)Vo (s)

= −Z f

Zi= −

(R2 + 1

sC1

) ‖ 1sC2

R1 ‖ (R3 + 1

sC3

) (41)

There are two well-known methods for designing atype III error amplifier. The first method is the K fac-tor method (Fernandez et al., 2013); the second method

(Raut & Talukder, 2011) is called manual placement ofpoles and zeros (MPPZ). In theMPPZapproach, the reso-nant frequency of the LC filter is used and given by (42).

fLC = 12π

√LC

(42)

The first zero is commonly placed at 50%–100% ofthe fLC, while the second zero is placed at fLC. The sec-ond pole is placed at the equivalent series resistance (ESR)zero in the filter transfer function (1/(rcc)); the third poleis placed at one-half of the switching frequency. The val-ues for the controller zeros and poles are as follows: ωz1=

Figure . DC-link voltage in steady-state condition.

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862 M. G. SIMÕES ET AL.

Figure . The three-phase PCC voltage and the WG three-phase output current in steady-state condition.

Table . Parameters of the type-III compensator.

C = . nF R = k�C = . nF R = k�C = uF R = �

461.2667, ωz2= 922.5334, ωp1= 0, ωp2= 5.1894k andωp3= 62.832k. Table 2 represents the parameters of thedesignated compensators.

Figure 16 presents the FIBC output voltage in steady-state conditionwhen the PV system is injecting 2 kW intothe grid. It is verified that the FIBC output voltage is con-trolled at 380 V.

.. Smart grid-connected inverter (SGCI)Controlling the injected power is the most importantissue during the grid-connected mode (Hu, Kuo, Lee, &

Guerrero, 2011). In order to achieve this goal, the cur-rent components are controlled. Due to the DC nature ofthe d − q components in the d–q reference frame, a PIcontroller is used to control the d and q components ofcurrents. Moreover, since the d and q control loops havethe same dynamics, the tuning of the PI controller for thecurrent is done only for the d-axis.

The reference values of currents are calculated from(43) and (44), where Pref and Qref are determined, forexample, by an energy management system in additionto power negotiation with the main grid.

Pref = 32

(VdId∗ +VqIq∗) (43)

Qref = 32

(VqId∗ −VdIq∗) (44)

Figure . Schematic of the proposed power electronic interface for grid-connected PV-based system and its control strategy.

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Figure . The FIBC output voltage in steady-state condition when the PV system is injecting kW into the grid.

Table 3 presents the FIBC parameters, while in Table 4the SGCI is presented. For this set of simulations, the ref-erence values for active and reactive powers are set to2 kW and 0 kVAR, respectively.

Table . Parameters of FIBC.

Input voltage Vin = VOutput voltage Vout = VInductor L= e- HCapacitor C= .e- FDuty cycle D= .Output capacitor ESR r= e-�

Switching frequency kHz

Figure 17 presents the three-phase PCC voltage andthe three-phase PV system output current. Similar to theWG system, the current is sinusoidal and in phase withthe PCC voltage.

Table . Parameters of the inverter.

Input voltage VDC= VOutput voltage (Ф) Vout = VrmsInductor of LCL L= e- HCapacitor of LCL C= .e- FSwitching frequency Fsw = KHzFrequency of the grid fg = Hz

Figure . The three-phase PCC voltage and the three-phase PV system output current in steady-state condition.

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864 M. G. SIMÕES ET AL.

Table . Prototype parameters.

RMS phase-voltage grid Vgrid = VGrid frequency fgrid = HzBBS output inductance LBBS = . mHInverter upper module voltage Vh = V (× V battery)Inverter middle module voltage Vm = V (× V battery)Inverter lower module voltage VL = V (× V battery)

3.3 Thewhole system

The three-phase system presented in Figure 1 was simu-lated. The parameters are presented in Table 5. The bat-teries were simulated by using the battery model found inthe PSIM software.

Figure 18 presents the PV, theWG and the BBS outputpower, as well as the sumof them for phaseA. The PV andthe WG supply a variable power due to the intermittentbehaviour found in renewable sources. At t = 1.5 s, theBBS begins to operate. It is noticeable that the BBS com-pensates for the deficiency of the PV and WG systems.Consequently, the grid receives constant power indepen-dently of wind and sun conditions. During the intervalbetween 3.2 and 4.9 s, the PV andWG both supply nom-inal power. Therefore, the BBS does not inject any power,and the BBS will charge its battery bank.

Figure 19 presents the three-phase PCC voltage andthe three-phase BBS output current under mode 2. TheBBS output current is sinusoidal and in phase with thePCC voltage.

Figure 20 presents the BBS terminal voltage for eachmodule for phase A. The upper waveform is the uppermodule terminal voltage. As previously described, thenumber of commutations is reduced, indicating a reduc-tion in the power losses.

Figure 21 presents the PCC voltage, the BBS currentand the grid current during the transition frommode 2 tomode 4. During this transition, the BBS begins to operateas an active filter in addition to providing injection power.Before t = 0.75 s, the BBS injects active power with unitypower factor. Its output current is sinusoidal and in phasewith the PCC voltage. Once there is a nonlinear load con-nected to the PCC and there is no filter to compensate theharmonic currents, the grid current is distorted. At t =0.75 s, the BBS begins to operate as an active filter simul-taneously with the injection power mode. As a result, itsoutput current is distorted because of the harmonic cur-rents and the grid current becomes sinusoidal. Now, oneof the smart features of the proposed BBS appears. Thegrid begins absorbing active power injected by PV, theWG and BBS, as can be observed in the same figure bythe 180° displacement between the PCC voltage and thegrid current. Even absorbing active power, the grid isreceiving active filter compensation by means of the BBS.

4. Experimental results

The system presented in Figure 1 has been partially veri-fied experimentally with a single-phase 2 kVA prototype.The experiment uses the same parameters as in simula-tion. Although the experimental set-up could not have afull WG and PV because of laboratory resource limita-tions, the BBS systemwas embedded with a fictitiousWGandPVoutput power curve behaviour. Such a curve emu-lates the information produced from theWG and PV sys-tems, and the curve demonstrates similar currents. As canbe observed, the experimental results demonstrate theefficacy of the BBS. The control strategy and the decision

Figure . The PV, the WG and the BBS output power, as well as the sum of them for phase A.

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Figure . Three-phase PCC voltage and the three-phase BBS output current in steady-state condition under mode .

algorithmwere embedded in the float-point digital signalcontroller TMS320L28335. The experimental results pre-sented in this section come from the flow chart presentedin Figure 3. The measured variables are referred to thosepresented in Figure 1.

Figure 22 presents the PCC voltage, BBS output cur-rent and the load current during transition from mode2 to mode 1. Initially, the price of the energy is the oneexpected, and the BBS output current is sinusoidal andin phase related to the PCC voltage, which means that

the BBS injects active power into the grid. Later, the BBSbegins to charge the batteries with the unity power factorfrom the grid’s point of view. As a result, the BBS outputcurrent is 180° phase-shift related to the PCCvoltage. Theload current is kept unchanged.

Figure 23 presents the BBS terminal voltage and thecell output voltages during mode 2 in a steady-state con-dition. The terminal voltage has 19 levels, and it pro-duces a sinusoidal waveform with a reduced output filter.The upper cell output voltage is a quasi-square waveform

Figure . BBS terminal voltage for each module for phase A.

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866 M. G. SIMÕES ET AL.

Figure . The PCC voltage, the BBS current and the grid current when the BBS during the transition frommode to mode .

with 60 Hz frequency. Since the commutation number inthis module is reduced, it is expected that a reduction inthe switching losses will be observed compared to othertopologies.

Figure 24 presents the BBS output current measuredon a long timescale and a detail of a short timescale

when the WG and PV systems are emulated. This oper-ation corresponds to mode 3 as previously explained.The BBS output current amplitude changes because it isinjecting power according to the difference between theWG and PV instantaneous output power. Therefore, thesum of the BBS and WG and the PV output powers is

Figure . PCC voltage, BBS output current and the load current during transition frommode to mode .

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Figure . The BBS terminal voltage (vt) and the cell output voltages (vt_h, vt_m and vt_L) (R, R, R, Ch: mV/ V).

Figure . The BBS output currentmeasured in a long timescale and a detail of a short periodwhen theWG and PV systems are emulated,mode (Ch: . V/A).

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868 M. G. SIMÕES ET AL.

Figure . PCC voltage, the BBS, the grid and load currents when the BBS is operating exclusively as active filter (mode ).

constant. The figure shows that the BBS output current issinusoidal.

Figure 25 shows the PCC voltage, the BBS, the gridand load currents when the BBS operates exclusively asan active filter (mode 6). The BBS output current is highlydistorted and the achieved grid current waveform is closeto the PCC voltage. This is because the CPT forces thegrid current to follow the PCCwaveform, similar to resis-tive load behaviour.

5. Conclusions

Connecting a storage system in parallel to both WG andPV systems is an effective solution for supplying constantpower to a systemwith a nonlinear load. Besides themainfunctions of the proposed smart battery-based system,the required active power is supplied with a sinusoidalcurrent waveform using CPT. The implemented asym-metric cascaded multilevel inverter shows the advan-tage of transferring the major part of the power under60 Hz switching frequency. Consequently, the commu-tation number is reduced and the power losses are min-imised as shown in both simulation and experimentalresults. Moreover, the dimensioning of the output fil-ter is reduced. Experimental results show a very effi-cient performance of the BBS. Neither unpredictabilitynor instability was observed. The proposed full-fledged

system described in this paper is a potential candi-date towards a novel solution of a hybrid storage sys-tem associated with WG and PV systems. The simula-tion file used in this paper will be freely available onhttps://sites.google.com/site/busarellosmartgrid/home.

Acknowledgements

The authors are grateful for the São Paulo Research Foundation(FAPESP), grant no. 2012/23914-5, as well as to CNPq grant no.303036/2010-9 and CAPES. Thanks to the US Department ofState Fulbright Fellowship allowing one of the authors to workin Denmark for 6 months.

Disclosure statement

No potential conflict of interest was reported by the authors.

ORCID

Marcelo Godoy Simões http://orcid.org/0000-0003-4124-061XFrede Blaabjerg http://orcid.org/0000-0001-8311-7412

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